This invention relates generally to radio receiver devices, and more particularly to radio receivers used as remote switching devices in communication systems such as cable television, and the like. Specifically, the present invention is directed to a code operated receiver which receives signal information in the form of a serial code and applies this information to a logic circuit which produces an output signal when the proper predetermined code for that logic circuit is received. The output signal from the logic circuit then actuates switching means to allow passage through the switching device of data information from a plurality of remotely located data transmitters back to a control station.
In communication systems, such as cable television, or the like, where data signal information is sent from a plurality of terminal points to a central data retriever it is often necessary to provide some means for identifying the particular terminal point so that the exact source of the data being received is known. To accomplish this the data transmission equipment at each terminal point may be costly, particularly if the number of terminal points is large. Therefore, it is desirable in large communication systems to reduce, as much as possible, the unit cost of each data transmitting device at each terminal point. A particular system arrangement for accomplishing this is disclosed in application Ser. No. 227,752, filed Feb. 22, 1972, now U.S. Pat. No. 3,786,424 and assigned to the same assignee of record. The disclosure within that application is incorporated herein by reference to the extent needed to understand the environment of the present invention.
CATV systems constitute one type of communications system that can be used for transmitting data over coaxial cables. These cable television systems often have a large number of subscribers constituting terminal points within the system where television receivers are located and are to be monitored. In the system disclosed in application Ser. No. 227,752 a plurality of television receivers make up a data transmitting group which may be identified by a particular code. A plurality of groups are provided in the system and each group has a different code. To allow access of the data transmitting devices at the television sets within each group a code operated switching device is utilized to be actuated in response to the code associated with that group to allow direct data transmission from the data transmitters located at the television sets back to the central station or head end. However, the system utilizes a plurality of code operated switches operated in response to RF frequency signals applied thereto. Each of the code operated switches includes an RF frequency receiver which functions in a superheterodyne fashion and therefore, require an oscillator to provide a frequency which is mixed with the incoming RF frequency to develope an intermediate frequency thereform. The code signal information is then detected, as for example, by FM detector means, to produce code signal pulses. These code operated switching devices operate at relative high frequencies, in the order of 100 to 200 MHz. Therefore, the relative frequency stability of local oscillators is critical for proper operation. Due to environmental changes such as changes in ambient temperature the local oscillator must be of extremely high quality design and must use high quality components to insure stability of operation throughout the temperature ranges that it is subjected to. This substantially increases the cost of each of the coded switching devices within a coaxial cable television system, and thereby increases the cost of the overall system.